Seasonal dynamics of phytoplankton and phytobenthos,and associated chemical interactions,in a shallow upland lake (Malham Tarn,northern England)

Seasonal changes of phytoplankton were followed over 3 years (1985–87) in a shallow, unstratified and calcareous upland lake.The phytoplankton was of low to moderate abundance and generally dominated by phytoflagellates. Seasonality involved a winter minimum of abundance, a spring maximum of diatoms, and often brief increases in summer that included blue-greens, especially the colonial Gloeotrichia echinulata. Some components were of benthic origin. Seasonal growth of the main component of the phytobenthos, Chara globularisvar. virgata, caused a regular summer depletion in lake water of Ca²⁺ and HCO₃ ^- (alkalinity) by associated CaCO₃ deposition, and a more extreme (and unusual) depletion of K⁺. Chemical analysis of Chara biomass and of underlying sediments indicated a large benthic nutrient stock, much surpassing that represented by the phytoplankton. Growth in this biomass, and the magnitude of water-borne inputs, influenced the removals of Ca²⁺, K⁺ and inorganic N. The phytoplankton was probably limited by a low-P medium, to which co-precipitation of phosphate with CaCO₃ may have contributed. A vernal depletion of Si was probably limiting to diatom growth, and appeared to be mainly induced by benthic rather than planktonic diatoms. Examples of long-term change in composition of the phytoplankton and phytobenthos are noted and discussed in relation to the interaction of these components, nutrient enrichment, and possible alternative stable states.     

Nutrient ratios and phytoplankton community structure in the large, shallow, eutrophic, subtropical Lakes Okeechobee (Florida, USA) and Taihu (China)

Analysis of ten- and four-year datasets for the large, shallow, subtropical, and eutrophic Lakes Okeechobee (USA) and Taihu (China), respectively, suggest that resource-ratio explanations for cyanobacteria dominance may not apply to these two lakes. Datasets were examined to identify relationships between nutrient ratios [total nitrogen (TN):total phosphorus (TP) and ammonium (NH₄ ⁺):oxidized N (NO _x )] and phytoplankton community structure (as proportions of cyanobacteria and diatoms to total phytoplankton biomass). Datasets were pooled by sampling month, averaged lake-wide, and analyzed with linear regression. In Okeechobee, the cyanobacteria proportion increased and the diatom proportion decreased with increasing TN:TP. In Taihu, cyanobacteria decreased with increasing TN:TP, but the opposite trend observed for diatoms was marginally significant. Okeechobee cyanobacteria increased and diatoms decreased with increasing NH₄ ⁺:NO _x , but no significant relationships between phytoplankton and NH₄ ⁺:NO _x were observed in Taihu. Both lakes had significant relationships between phytoplankton community structure and total nutrients, but these relationships were the opposite of those expected. Relationships between phytoplankton community structure and water quality parameters from the previous month resulted in improved relationships, suggesting a predictive capability. Statistical analysis of the entire datasets (not pooled) supported these and additional relationships with other parameters, including temperature and water clarity.     

Species composition and phytoplankton biomass in a tropical African lake (Lake Awassa,Ethiopia)

The species composition and phytoplankton biomass of Lake Awassa, Ethiopia were studied from September 1985 to July 1986 in relation to some limnological features of the lake. During the study period, three phases of thermal stratification were recognized: a period of unstable stratification and near-complete mixing was followed by a stable stratification period and another period of complete mixing. Complete mixing was associated with cooling of air temperature with an influx of cool rain and high rainfall. The underwater light penetration showed a similar pattern over the whole period with the highest in the red, and the lowest in the blue spectral region. Euphotic depth varied between 1.6 and 3.0 meters with the highest measurements corresponding to the stable stratification period. PO₄-P concentrations ranged between 23 and 45 µg l^–1 and NO₃-N concentrations varied between 7 and 14 µg l^–1 during the study period. Both nutrients showed increasing values associated with mixing periods and/or the rainy season.A total of 100 phytoplankton species were identified with 48% of the taxa represented by green algae, 30% by blue-green algae, 11% by diatoms, and the rest by chrysophytes, dinoflagellates, cryptomonads and euglenoids. The dominant phytoplankton species were Lyngbya nyassae, Botryococcus braunii and Microcystis species. Seasonal biomass variation was pronounced in the first two species but not in Mycrocystis. Phytoplankton biomass increased following the mixing period in December, and thermal destratification during May to July which was also a period with high rainfall and relatively high nutrient concentration. While the seasonal variation of the total phytoplankton community in Lake Awassa was relatively low (coefficient of variation < 20%), it was higher in some of the individual component species.     

Photosynthetic Activity of Dominant Algal Species in Eutrophic Shallow Lake (Großer Müggelsee,Berlin) Investigated by Microautoradiography

The photosynthetic activity of dominant phytoplankton in eutrophic shallow lake was investigated by the autoradiographic method in 1979 and 1980. It was shown by light and dark field microscopy that all species of Cyanophyta (Oscillatoria redekei, Oscillatoria agardhii, Aphanizomenon flos-aquae) were characterized by a continuously high uptake of NaH¹⁴CO₃. Similarly high photosynthetic activity was observed during the occurrence of Cryptomonas sp. and nanoplankton. Contrary to these observations, diatoms showed remarkably high portions of photosynthetically inactive biomass when their development was abundant. The reasons for this discrepancy between high biomass of diatoms and relatively low primary production (measured by the ¹⁴C-method and autoradiography) are discussed.     

Composition and variation of phytoplankton communities during Microcystis bloom in an artificial lagoon of Hangzhou Bay,China

The interaction of various environmental triggers on phytoplankton communities of an artificial lagoon of Hangzhou Bay China, was studied during a Microcystis bloom in summer 2016. Forty-two phytoplankton genera (six phyla) were defined, with Bacillariophyta accounting for half of all phytoplankton genera. It was determined that Melosira, Chlorella, Cyclotella, Microcystis, Merismopedia, Anabaena and Selenastrum, which were identified and counted by an inverted microscope, were the dominant genera. In addition, a series of environmental indicators were analyzed, including salinity, seawater temperature, dissolved inorganic nitrogen, soluble reactive phosphorus (PO₄-P), ammonium (NH₄-N), nitrate nitrogen (NO₃-N), nitrite (NO₂-N), silicate (SiO₄-Si), and chemical oxygen demand of the water samples, as well as zooplankton community. The results of variance partitioning by R language revealed that the most influential factor driving the change in the phytoplankton community was the environment (49.7%), and zooplankton grazing represented only 7.9%. The results of redundancy analysis indicated that the change and composition of the phytoplankton community correlated significantly with the interaction of salinity, PO₄-P, transparency, seawater temperature, and the dominant zooplankton species. Notably, salinity and temperature fluctuation were the key factors inducing the rapid succession of the plankton community in artificial lagoons such as within the Jinshan City Beach (Shanghai, China).

     

Food concentration and temperature effects on the demography of <Emphasis Type="Italic">Latonopsis</Emphasis> cf. <Emphasis Type="Italic">australis</Emphasis> Sars (Cladocera: Sididae)

This study has two main objectives, the first being the determination of net phytoplankton primary production to explain the phytoplankton’s function in a wetland carbon cycle, while the second objective is to relate this function with the phytoplankton assemblage composition. The annual variation in the phytoplankton production was monitored monthly for more than a year (2007–2008) in the semiarid eutrophic, hydrologically-perturbed “Tablas de Daimiel” National Park wetland. The phytoplankton fraction considered in this study comprised all organisms between the size 3 and 100 μm. The total biomass of phytoplankton was obtained by counting algae and calculating their volume, while net primary production and respiration were quantified by in situ incubations with the Winkler method. The respiration ranged from undetectable to 0.07 mgO₂ l⁻¹ h⁻¹; net photosynthesis reached 0.20 mgO₂ l⁻¹ h⁻¹. Net primary production was maximum at the end of the warm period (October 2007), and other peaks occurred at the start of summer (July 2007) or spring (March 2008). When maximum production took place, phytoplankton was mainly composed of small fast-growing chlorophytes (Tetraselmis cf. fontiana or Chlamydomonas sp.), in addition to some of the large, S-strategist algae (Peridinium umbonatum, Microcystis flos-aquae, Euglena sp.). The phytoplankton metabolism in “Tablas de Daimiel” was autotrophic as a whole due to changing contributions of algal groups. Only chlorophyte biomass was statistically related to net primary production. The conclusion reached is that this shallow eutrophic semiarid wetland possessed an annual net autotrophic production of phytoplankton fraction resulting from the small, fast-growing algae enhanced by hydrological perturbations that interrupted the autogenic course of S strategists.     

Cloning,Expression and Characterization of the δ‐carbonic Anhydrase of Thalassiosira weissflogii (Bacillariophyceae)

Carbonic anhydrase (CA) is a ubiquitous metalloenzyme responsible for accelerating the interconversion of CO₂ and bicarbonate. Although CAs are involved in a broad range of biochemical processes involving carboxylation or decarboxylation reactions, they are of special interest due to their role in photosynthetic CO₂ assimilation in marine phytoplankton, especially under low‐CO₂ conditions. Several phylogenetically independent classes of CAs have been identified in a variety of marine phytoplankton. TWCA1, first discovered in Thalassiosira weissflogii (Grunow) G. Fryxell & Hasle, is the founding member of the δ‐class of CAs; these appear to be extracellular enzymes, but are still relatively poorly characterized. To date, it has remained uncertain whether TWCA1 possesses true CA activity due to the difficulty in producing a functional protein in a heterologous expression system. Herein we describe the fusion of a full‐length open reading frame of TWCA1 to the coding sequence of a self‐splicing intein in a pTWIN2 expression vector that has allowed successful production of a functional enzyme in Escherichia coli. Assay of the recombinant protein shows that TWCA1 is a catalytically active δ‐CA possessing both CO₂ hydration and esterase activity.     

Seasonal variation in phytoplankton primary production in relation to light and nutrients in Lake Awasa,Ethiopia

The biomass and primary production of phytoplankton in Lake Awasa, Ethiopia was measured over a 14 month period, November 1983 to March 1985. The lake had a mean phytoplankton biomass of 34 mg chl a m^–3 (n = 14). The seasonal variation in phytoplankton biomass of the euphotic zone (mg chl a m^–2 h^–1) was muted with a CV (standard deviation/mean) of 31%. The vertical distribution of photosynthetic activity was of a typical pattern for phytoplankton with light inhibition on all but overcast days. The maximum specific rates of photosynthesis or photosynthetic capacity (Ø_max) for the lake approached 19 mg O₂ (mg chl a)^–1 h^–1, with high values during periods of low phytoplankton biomass. Areal rates of photosynthesis ranged between 0.30 to 0.73 g O₂ m^–2 h^–1 and 3.3 to 7.8 g O₂ m^–2 d^–1. The efficiency of utilisation of PhAR incident on the lake surface varied from 2.4 to 4.1 mmol E^–1 with the highest efficiency observed corresponding to the lowest surface radiation. Calculated on a caloric basis, the efficiency ranged between 1.7 and 2.9%. The temporal pattern of primary production by phytoplankton showed limited variability (CV = 21 %).     

Photosynthetic activity of phytoplankton in a high altitude lake (Emerald Lake,Sierra Nevada,California)

Photosynthetic activity by phytoplankton was measured during the ice-free seasons of 1984, 1985 and 1987 using the ¹⁴C radioassay in high altitude Emerald Lake (California). Relative quantum yield (^B) and light-saturated chlorophyll-specific carbon uptake (P_m ^B) were calculated from the relationship of light and photosynthesis fitted to a hyperbolic tangent function. Temporal changes in P_m ^B showed no regular pattern. Seasonal patterns of ^B generally had peaks in the summer and autumn. Phytoplankton biomass (as measured by chlorophyll a) and light-saturated carbon uptake (P_m) had peaks in the summer and autumn which were associated with vertical mixing. Estimates of mean daily carbon production were similar among the three years: 57 mg C m^–2 2 d^–1 in 1984, 70 mg C m^–2 2 d^–1 in 1985 and 60 mg C m^–2 d^–1 in 1987. Primary productivity in Emerald Lake is low compared to other montane lakes of California and similar to high-altitude or high-latitude lakes in other regions.     

Cryptic speciation in the cosmopolitan <Emphasis Type="Italic">Epiphanes senta</Emphasis> complex (Monogononta,Rotifera) with the description of new species

The phytoplankton species composition and seasonal succession were examined in Lake Kastoria during the period November 1998–October 1999. A total of 67 species and 19 functional groups were identified. Only 4 out of the 67 species, all Cyanobacteria, were dominant (Limnothrix redekei, Microcystis aeruginosa, Cylindrospermopsis raciborskii and Aphanizomenon gracile). Diatoms were rare, not only in terms of species number, but also in terms of biomass (contributing <5% to the total phytoplankton biomass) in relation to the rather low silicon concentrations throughout the year. The functional groups S1, S_N, M and H1 were found dominant in the lake. The species A. gracile (functional group H1) behaved like the species Cylindrospermopsis raciborskii (functional group S_N) which is tolerant to mixing and poor light conditions. The phytoplankton seasonal succession showed similar patterns in all six sampling stations, both at the surface and the bottom water layer, with minor differences during Microcystis aeruginosa dominance. Two steady-state phases were identified within a year lasting for 4 months under relatively stable physical conditions. In these steady-states, the Limnothrix redekei persistent dominance under low light availability and low inorganic nitrogen has been explained by its specific ability such as buoyancy regulation to exploit resources in the water column. Moreover, high population densities over the winter and before the development of daphnids may contribute to the steady-state dominance of Limnothrix. Different niches separated vertically in the water column is one of the explanations for the Limnothrix–Microcystis steady-state when a replacement between the two species was observed in different water layers and areas of the lake. Long lasting steady-states of Cyanobacteria observed in Lake Kastoria and in other Mediterranean and tropical freshwaters may indicate influence of warm climate properties on phytoplankton dynamics. Electronic Supplementary Material Supplementary material is available to authorized users in the online version of this article at http://dx.doi.org/10.1007/s10750-006-0360-4 Handling editor: K. Martens     

Vertical distribution of phytoplankton in a clear water lake of Colombian Amazon (Lake Boa, Middle Caquetá)

Vertical distribution of phytoplankton is highly influenced by physical and chemical factors, but the knowledge about these aspects remain unknown in Colombian Amazon lakes. In this work, the relations between the physico-chemical variables and the vertical distribution of phytoplankton community of a clear water lake (Lake Boa) are analyzed. Samples were taken at every 30 cm from surface to bottom in the center of the lake at different times of the hydrological cycle. Phytoplankton transformed from a diluted community dominated by cyanobacteria in high waters to one with elevated abundance of coccal chlorophytes in low waters. In the low water phase, the vertical distribution of Cryptomonas spp., Peridinium cf. umbonatum and Chlamydomonas spp. had significant statistical associations with the redox potential (RP), reactive phosphorus (PO₄), and hydrogen sulfide (H₂S) (p ≤ 0.05). During the flood stage the conditions were more varied and significant associations were detected between the vertical arrangement of Botryococcus braunii, Oscillatoria splendida, Dinobryon serturlaria, Euglena acus, and Trachelomonas volvocina with conductivity, dissolved oxygen (DO), pH, temperature, dissolved organic carbon (DOC), PO₄, RP, and ammonia (NH₄). The vertical structure of the Lake Boa’s phytoplankton responds to annual changes produced by the flood pulse, and to the vertical environmental variations, which are stronger during the high water phase.     

Effects of the halocline on water quality and phytoplankton composition in a shallow brackish lake (Lake Obuchi, Japan)

The relationships of the halocline to both water quality and phytoplankton composition in Lake Obuchi, a shallow brackish lake in northern Japan, were investigated from April 2001 to December 2004. The halocline in this lake became stronger in summer (July–September, mean maximum density gradient 4.3–5.8 ρ_tm⁻¹) but weaker in spring, fall, and winter (1.9–3.3 ρ_tm⁻¹). Although the difference in water quality between the upper and lower layers separated by the halocline was high in summer, nutrients (PO₄³⁻-P and NH₄⁺-N) were eluted from the bottom sediment as levels of dissolved oxygen decreased in the bottom layer because of the strong stratification caused by the halocline formed over the long term. Moreover, phytoplankton taxa composition also differed between the upper and lower layers in summer, but was similar in other seasons. The dominant phytoplankton taxa in the upper layer in summer were Skeletonema costatum and Cyclotella spp., whereas in the lower layer, Gymnodinium spp. (Dinophyceae) and Chlorophyceae, which prefer eutrophic and low dissolved oxygen conditions, dominated. This suggests that the halocline was related to differentiations in both water quality and ecosystem components between the upper and lower layers in the brackish lake water.     

Ecology of Dictyosphaerium pulchellum Wood (Chlorophyta, Chlorococcales) in a shallow, acid, forest lake

This study relates to the ecology of Dictyosphaerium pulchellum Wood in Delamere Lake in Cheshire, UK. Dictyosphaerium pulchellum is a cosmopolitan, green colonial phytoplankton species that occasionally forms dense, monospecific populations in lakes. Delamere Lake is a small, shallow, acid lake (mean pH, 4.5) with very high phytoplankton biomass (annual mean chlorophyll a, 290 μg l⁻¹) and devoid of any significant cladoceran population, the efficient grazers of phytoplankton. A predominantly unicellular form of D. pulchellum was the dominant species in Lake Delamere, and it comprised on average ca. 80% (maximum >99%) of the lake phytoplankton biovolume. Laboratory and lake experiments were conducted on this species showed that its pH tolerance varied between 2.4 and 10.7, and its optimum tolerance range between 3.3 and 8.5 depending on other environmental variables. Low pH was not responsible for the unicellular habit of this alga, but a very high nutrient regime could be an important factor. Bioassays revealed that in Delamere Lake this species was limited by nitrogen, but nitrogen did not hamper high growth in the lake. Dictyosphaerium pulchellum can persist at low light levels, tolerate CO₂-deficiency and can grow in polyhumic water with water colour around 300 mg Pt l⁻¹, but probably not in darker waters. The dominance of D. pulchellum in Delamere Lake is apparently due to a combination of several factors: its ability to tolerate both low pH and high turbidity, exploit high nutrient conditions, absence of effective grazing pressure by zooplankton and being a superior competitor.     

OSCILLATORIA (TRICHODESMIUM) THIEBAUTII (CYANOPHYTA) IN THE CENTRAL NORTH ATLANTIC OCEAN12

Physiological rate measurements were made with Oscillatoria thiebautii (Gom.) Geitler in the subtropical north Atlantic Ocean between Spain and Bermuda during May and June of 1975. The near surface C:N fixation ratios averaged 6.5, and the cellular composition ratio was 6.2, suggesting that N₂ fixation is the major path of nitrogenous nutrition for this alga. Compared to other oceanic phytoplankters, it has a low affinity for orthophosphate at oceanic concentrations (k_s= 9.0); however, it has a high potential for utilizing phosphomonoesters (170–300 ng atoms P ·μg chl a^?1· h^?1). Maximal photosynthesis occurred at 450–700 μ Einstein · m^?2· s^?1, and was inhibited by full sunlight. Calculated cell division rates (ca. 180 days) suggest that relative to other phytoplankters in this oceanic region, O. thiebautii must be subjected to negligible grazing pressure. No major differences in C, N, chl a or ATP were observed between the tuft (fusiform) and puff (spherical) colonies. ATP concentrations relative to other cellular constituents varied greatly between colonies, suggesting a general inter-colony physiological variability in the open Atlantic. With increasing depth in the euphotic zone, there was no evidence for chromatic adaption. The observations that O. thiebautii represents only a small fraction of total phytoplankton biomass and that its growth rate is 10–100 times slower than that of the other indigenous phytoplankton, strongly suggest that N₂ fixation by this alga is a virtually insignificant component of the nitrogenous nutrition for the phytoplankton of the North Atlantic central gyre in late Spring.     

Hydraulic Phases, Persistent Stratification, and Phytoplankton in a Tropical Floodplain Lake (Mary River, Northern Australia)

The Mary River, in the Australian wet/dry tropics, flows seasonally. When the river ceases flowing in the dry season, a series of isolated lakes remain along the river’s main floodplain channel. The limnology of a channel lake, which is 14 km long and 6-9 m deep in the dry season, was examined between April and December 2000. Four hydraulic phases were identified, these being (1) riverine (April), (2) riverine to lake transition (May), (3) lake (June–late-November), and (4) lake to riverine transition (late-November–December). These phases differ with respect to their duration and flow direction from lakes located on tropical floodplains of perennially flowing rivers. Despite the variable hydraulic conditions, the main channel remained thermally stratified, with only infrequent and short-lived deep mixing events, and sufficient light for photosynthesis in the diurnal mixed layer. During the period of isolation and in contrast to floodplain lakes in tropical South America, the depth of the Mary River channel lake always exceeded, by at least 2-fold, the depth of the diurnal mixed layer. The water quality (conductivity, dissolved oxygen, pH, Si and water clarity) and phytoplankton assemblage of the channel lake was primarily driven by its hydraulics, though this was not evident for the channel’s nutrient concentrations. Dissolved oxygen concentrations during lentic conditions were double values during the riverine and transition phases. This was attributed to the cessation of inflowing waters with a high biological oxygen demand, and enhanced photosynthetic activity of higher concentrations of phytoplankton retained under lentic conditions. The channel’s phytoplankton assemblage reflected the channel’s hydraulics, with the most common phytoplankton throughout the study period belonging to functional groups L_o(Peridinium inconspicuum), W1 (euglenoids), W2 (Trachelmonas) and Y (Cryptopmonas, Rhodomonas), with groups A (Acanthoceras) and D (Nitzschia agnita, Synedra alna) prominent during the lentic phase. Despite persistent stratification under lentic conditions, there was no clear evidence of autogenic succession or domination by any single phytoplankton functional group.     

Metabolic rates during rest and activity in differently tracheated spiders (Arachnida,Araneae):<Emphasis Type="Italic"> Pardosa lugubris</Emphasis> (Lycosidae) and<Emphasis Type="Italic"> Marpissa muscosa</Emphasis> (Salticidae)

With flow-through respirometry under video tracking, the CO₂ release of adult male and female Pardosa lugubris (wolf spider) and Marpissa muscosa (jumping spider) was measured during rest and activity. Activity metabolism was measured in phases in which the animals were spontaneously active and during forced exercise. Standard metabolic rates (V_CO2/t) were 1.43 nmol s^–1 g^–1 in M. muscosa and 1.7–1.8 nmol s^–1 g^–1 in P. lugubris. Egg production caused higher resting rates in females compared with the males in P. lugubris. Maximum mass-specific CO₂ release, the additional amount of CO₂ released after activity and the factorial aerobic scope were higher in M. muscosa. Additionally, half-time recovery and the lag between end of activity and maximum CO₂ release were lower in the jumping spider. The results are consistent with the hypothesis that the well-developed tracheal system in jumping spiders increases the efficiency of the respiratory system in comparison with wolf spiders, which possess similarly developed lungs but only a simple tracheal system that is restricted to the opisthosoma.Communicated by G. Heldmaier     

Phytoplankton and primary production in clear-vegetated,inorganic-turbid,and algal-turbid shallow lakes from the pampa plain (Argentina)

Shallow lakes often alternate between two possible states: one clear with submerged macrophytes, and another one turbid, dominated by phytoplankton. A third type of shallow lakes, the inorganic turbid, result from high contents of suspended inorganic material, and is characterized by low phytoplankton biomass and macrophytes absence. In our survey, the structure and photosynthetic properties (based on ¹⁴C method) of phytoplankton were related to environmental conditions in these three types of lakes in the Pampa Plain. The underwater light climate was characterized. Clear-vegetated lakes were more transparent (K _d 4.5–7.7 m⁻¹), had high DOC concentrations (>45 mg l⁻¹), low phytoplankton Chl a (1.6–2.7 μg l⁻¹) dominated by nanoflagellates. Phytoplankton productivity and photosynthetic efficiency (α ~ 0.03 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²) were relatively low. Inorganic-turbid lakes showed highest K _d values (59.8–61.4 m⁻¹), lowest phytoplankton densities (dominated by Bacillariophyta), and Chl a ranged from 14.6 to 18.3 μg l⁻¹. Phytoplankton-turbid lakes showed, in general, high K _d (4.9–58.5 m⁻¹) due to their high phytoplankton abundances. These lakes exhibited the highest Chl a values (14.2–125.7 μg l⁻¹), and the highest productivities and efficiencies (maximum 0.56 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²). Autotrophic picoplankton abundance, dominated by ficocianine-rich picocyanobacteria, differed among the shallow lakes independently of their type (0.086 × 10⁵–41.7 × 10⁵ cells ml⁻¹). This article provides a complete characterization of phytoplankton structure (all size fractions), and primary production of the three types of lakes from the Pampa Plain, one of the richest areas in shallow lakes from South America. Handling editor: J. Padisak     

PHOTOSYNTHESIS-IRRADIANCE RELATIONSHIPS IN PHYTOPLANKTON FROM THE PHYSICALLY STABLE WATER COLUMN OF A PERENNIALLY ICE-COVERED LAKE (LAKE BONNEY,ANTARCTICA)1

The perennially ice-covered lakes of Antarctica have hydrodynamically stable water columns with a number of vertically distinct phytoplankton populations. We examined the photosynthesis-irradiance characteristics of phytoplankton from four depths of Lake Bonney to determine their physiological condition relative to vertical gradients in irradiance and temperature. All populations studied showed evidence of extreme shade adaptation, including low I_k values (15–45 μE · m^?2· s^?1) and extremely low maximal photosynthetic rates (P^B_m less than 0.3 μg C ·μg chl a^?1· h^?1). Photosynthetic rates were controlled by temperature as well as light variations with depth. Lake Bonney has an inverted temperature profile within the trophogenic zone that increased from 0° C at the ice-water interface to 6° C from 10 to 18 m. Deeper phytoplankton (10 m and 17 m) were found to have photosynthetic capacities (P^B_m) and efficiences (α) three to five times higher than those at the ice-water interface. However, Q₁₀ values were only ca. 2 for P^B_m (no temperature dependence was evident for α), suggesting that a simple temperature response cannot explain all the differences between populations. Lake Bonney phytoplankton (primarily cryptophytes and chlorophytes) had photosynthetic characteristics similar to diatoms from other physically stable environments (e.g. sea ice, benthos) and may be ecologically analogous to multiple deep chlorophyll maxima.     

Spatio‐temporal distribution of phytoplankton in four coastal rivers of southeastern of Ivory Coast (Soumié, Eholié, Ehania and Noé)

Spatio‐temporal dynamics of phytoplankton and their relation to abiotic environmental factors in four rivers of south‐eastern Ivory Coast (Soumié, Eholié, Ehania and Noé) was analysed from July 2003 to March 2005. The pelagic zone of each river was sampled upstream and downstream. Phytoplankton abundance was higher in Noé River (154.3 10⁴ cells l^?1) and lower in Eholié river (23.05 10⁴ cells l^?1). Dominant taxa were Microcystis aeruginosa (Kütz.) Lemmerm. and Aphanocapsa incerta (Lemmerm.) Cronberg & Komárek. In general, minimum densities of phytoplankton were observed during the rainy season, while maximum were observed in dry season in the whole stations of the rivers studied, such periods corresponding to low and high concentrations of nitrates. Among the rivers surveyed, Eholié river seems to be the least disturbed because of its higher species diversity. Seasonality fluctuations of algae abundance appear to be influenced by the flow of water and nitrate levels. This work is a useful starting point for future research on micro algae in Ivory Coast.